Photographic elements which produce images having an optical density directly related to the radiation received on exposure are said to be negative-working. A positive photographic image can be formed by producing a negative photographic image and then forming a second photographic image which is a negative of the first negative--that is, a positive image. A direct-positive image is understood in photography to be a positive image that is formed without first forming a negative image. Direct-positive photography is advantageous in providing a more straight-forward approach to obtaining positive photographic images.
A conventional approach to forming direct-positive images is to use photographic elements employing internal latent image-forming silver halide grains. After imagewise exposure, the silver halide grains are developed with a surface developer--that is, one which will leave the latent image sites within the silver halide grains substantially unrevealed. Simultaneously, either by uniform light exposure or by the use of a nucleating agent, the silver halide grains are subjected to development conditions that would cause fogging of a negative-working photographic element. The internal latent image-forming silver halide grains which received actinic radiation during imagewise exposure develop under these conditions at a slow rate as compared to the internal latent image-forming silver halide grains not imagewise exposed. The result is a direct-positive silver image. In color photography, the oxidized developer that is produced during silver development is used to produce a corresponding direct-positive dye image. Multicolor direct-positive photographic images have been extensively investigated in connection with image transfer photography.
Direct-positive internal latent image-forming emulsions can take the form of halide-conversion type emulsions. Such emulsions are illustrated by Knott et al U.S. Pat. No. 2,456,943 and Davey et al U.S. Pat. No. 2,592,250.
More recently the art has found it advantageous to employ core-shell emulsions as direct positive internal latent image-forming emulsions. An early teaching of core-shell emulsions is provided by Porter et al U.S. Pat. No. 3,206,313, wherein a coarse grain monodispersed chemically sensitized emulsion is blended with a finer grain emulsion. The blended finer grains are Ostwald ripened onto the chemically sensitized larger grains. A shell is thereby formed around the coarse grains. The chemical sensitization of the coarse grains is "buried" by the shell within the resulting core-shell grains. Upon imagewise exposure latent image sites are formed at internal sensitization sites and are therefore also internally located. The primary function of the shell structure is to prevent access of the surface developer to the internal latent image sites, thereby permitting low minimum densities.
The chemical sensitization of the core emulsion can take a variety of forms. One technique is to sensitize the core emulsion chemically at its surface with conventional sensitizers, such as sulfur and gold. Atwell et al U.S. Pat. No. 4,035,185 teaches that controlling the ratio of middle chalcogen to noble metal sensitizers employed for core sensitization can control the contrast produced by the core-shell emulsion. Another technique that can be employed is to incorporate a metal dopant, such as iridium, bismuth, or lead, in the core grains as they are formed.
The shell of the core-shell grains need not be formed by Ostwald ripening, as taught by Porter et al, but can be formed alternatively by direct precipitation onto the sensitized core grains. Evans U.S. Pat. Nos. 3,761,276, 3,850,637, and 3,923,513 teach that further increases in photographic speed can be realized if, after the core-shell grains are formed, they are surface chemically sensitized. Surface chemical sensitization is, however, limited to maintain a balance of surface and internal sensitivity favoring the formation of internal latent image sites.
Direct-positive emulsions exhibit art-recognized disadvantages as compared to negative-working emulsions. Although Evans, cited above, has been able to increase photographic speeds by properly balancing internal and surface sensitivities, direct-positive emulsions have not achieved photographic speeds equal to the faster surface latent image forming emulsions. Second, direct-positive core-shell emulsions are limited in their permissible exposure latitude. When exposure is extended, rereversal occurs. That is, in areas receiving extended exposure a negative image is produced. This is a significant limitation to in-camera use of direct-positive photographic elements, since candid photography does not always permit control of exposure conditions. For example, a very high contrast scene can lead to rereversal in some image areas.
A schematic illustration of rereversal is provided in FIG. 1, which plots density versus exposure. A characteristic curve (stylized to exaggerate curve features for simplicity of discussion) is shown for a direct-positive emulsion. When the emulsion is coated as a layer on a support, exposed, and processed, a density is produced. The characteristic curve is the result of plotting various levels of exposure versus the corresponding density produced on processing. At exposures below level A underexposure occurs and a maximum density is obtained which does not vary as a function of exposure. At exposure levels between A and B useful direct-positive imaging can be achieved, since density varies inversely with exposure. If exposure occurs between the levels indicated by B and C, overexposure results. That is, density ceases to vary as a function of exposure in this range of exposures. If a subject to be photographed varies locally over a broad range of reflected light intensities, a photographic element containing the direct-positive emulsion can be simultaneously exposed in different areas at levels less than A and greater than B. The result may, however, still be aesthetically pleasing, although highlight and shadow detail of the subject are both lost. If it is attempted to increase exposure for this subject, however, to pick up shadow detail, the result can be to increase highlight exposure to levels above C. When this occurs, rereversal is encountered. That is, the areas overexposed beyond exposure level C appear as highly objectionable negative images, since density is now increasing directly with exposure. Useful exposure latitude can be increased by more widely separating exposure levels A and B, but this is objectionable to the extent that it reduces contrast below optimum levels for most subjects. Therefore reduction in rereversal is most profitably directed to increasing the separation between exposure levels B and C so that overexposed areas are less likely to produce negative images. (In actual practice the various segments of the characteristic curve tend to merge more smoothly than illustrated.)
The use of inorganic salts of cadmium, manganese, and zinc as antifoggants is taught by Jones U.S. Pat. No. 2,839,405 and Sidebotham U.S. Pat. No. 3,488,709. Milton U.S. Pat. No. 3,761,266 teaches immersing a photographic element containing a core-shell emulsion having its shell comprised of silver chloride in a surface-image stabilizer bath containing cadmium chloride. Atwell U.S. Pat. No. 4,269,927 teaches that low levels of cadmium, lead, zinc, or copper dopants will increase the sensitivity of negative-working silver chloride emulsions.